This invention relates generally to refrigeration devices such as heat pumps and, in particular, to apparatus and a method for protecting a compressor in a heat pump.
Heat pumps typically include a compressor for compressing refrigerants, e.g. freon. The compressor has a crankcase containing lubricating oil and refrigerant, and a heater is provided for keeping the lubricating oil in the crankcase warm enough to prevent accumulation of any liquid refrigerant in the compressor. Proper operation of the crankcase heater is important because if the refrigerant is in a liquid state when the compressor starts, the compressor could fail resulting in a very expensive repair job. It is believed that many compressor failures in heat pumps result from failure of the crankcase heater.
U.S. Pat. No. 4,004,431 granted Jan. 25, 1977 to T. L. Hildreth discloses a cooling unit having an evaporator fan, a compressor, and a heater for the compressor. A control circuit which includes a thermostat that senses temperature changes in the compressor housing prevents actuation of the compressor until the heater raises the temperature of the compressor to a predetermined level. The control circuit also shuts down the compressor if the temperature of the compressor falls a predetermined amount or reaches a predetermined point. A drawback of the Hildreth control circuit is that the thermostat must be mounted directly on the compressor housing in order to accurately sense temperature changes. Another drawback of the Hildreth control circuit is that cold air flow from the evaporator fan may be detected by the thermostat thereby causing the compressor to be shut down inadvertently.
U.S. Pat. No. 3,577,741 granted May 4, 1971 to D. N. Shaw discloses a refrigeration unit including a compressor with a crankcase, a heating element on the compressor crankcase, and a control circuit for preventing energization of the compressor if the heating element is not functioning. The control circuit includes relays and switches that cooperate to energize and deenergize the compressor depending upon the condition of the heating element. A drawback of the Shaw control circuit is that it is too complex requiring major changes in a conventional refrigeration unit such as a heat pump. For example, it reduces the output of the heating element thus requiring the installation of a larger heating element which has a higher operating cost.